You need to say “yes” to ‘Magic SysRq key (CONFIG_MAGIC_SYSRQ)’ when
configuring the kernel. When running a kernel with SysRq compiled in,
/proc/sys/kernel/sysrq controls the functions allowed to be invoked via
the SysRq key. The default value in this file is set by the
CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbol, which itself defaults
to 1. Here is the list of possible values in /proc/sys/kernel/sysrq:

Some
keyboards may not have a key labeled ‘SysRq’. The ‘SysRq’ key is
also known as the ‘Print Screen’ key. Also some keyboards cannot
handle so many keys being pressed at the same time, so you might
have better luck with press Alt, press SysRq,
release SysRq, press <commandkey>, release everything.

On SPARC - You press ALT-STOP-<commandkey>, I believe.

On the serial console (PC style standard serial ports only)

You send a BREAK, then within 5 seconds a command key. Sending
BREAK twice is interpreted as a normal BREAK.

Will immediately reboot the system without syncing or unmounting
your disks.

c

Will perform a system crash by a NULL pointer dereference.
A crashdump will be taken if configured.

d

Shows all locks that are held.

e

Send a SIGTERM to all processes, except for init.

f

Will call the oom killer to kill a memory hog process, but do not
panic if nothing can be killed.

g

Used by kgdb (kernel debugger)

h

Will display help (actually any other key than those listed
here will display help. but h is easy to remember :-)

i

Send a SIGKILL to all processes, except for init.

j

Forcibly “Just thaw it” - filesystems frozen by the FIFREEZE ioctl.

k

Secure Access Key (SAK) Kills all programs on the current virtual
console. NOTE: See important comments below in SAK section.

l

Shows a stack backtrace for all active CPUs.

m

Will dump current memory info to your console.

n

Used to make RT tasks nice-able

o

Will shut your system off (if configured and supported).

p

Will dump the current registers and flags to your console.

q

Will dump per CPU lists of all armed hrtimers (but NOT regular
timer_list timers) and detailed information about all
clockevent devices.

r

Turns off keyboard raw mode and sets it to XLATE.

s

Will attempt to sync all mounted filesystems.

t

Will dump a list of current tasks and their information to your
console.

u

Will attempt to remount all mounted filesystems read-only.

v

Forcefully restores framebuffer console

v

Causes ETM buffer dump [ARM-specific]

w

Dumps tasks that are in uninterruptable (blocked) state.

x

Used by xmon interface on ppc/powerpc platforms.
Show global PMU Registers on sparc64.
Dump all TLB entries on MIPS.

y

Show global CPU Registers [SPARC-64 specific]

z

Dump the ftrace buffer

0-9

Sets the console log level, controlling which kernel messages
will be printed to your console. (0, for example would make
it so that only emergency messages like PANICs or OOPSes would
make it to your console.)

Well, unraw(r) is very handy when your X server or a svgalib program crashes.

sak(k) (Secure Access Key) is useful when you want to be sure there is no
trojan program running at console which could grab your password
when you would try to login. It will kill all programs on given console,
thus letting you make sure that the login prompt you see is actually
the one from init, not some trojan program.

Important

In its true form it is not a true SAK like the one in a
c2 compliant system, and it should not be mistaken as
such.

It seems others find it useful as (System Attention Key) which is
useful when you want to exit a program that will not let you switch consoles.
(For example, X or a svgalib program.)

reboot(b) is good when you’re unable to shut down. But you should also
sync(s) and umount(u) first.

crash(c) can be used to manually trigger a crashdump when the system is hung.
Note that this just triggers a crash if there is no dump mechanism available.

sync(s) is great when your system is locked up, it allows you to sync your
disks and will certainly lessen the chance of data loss and fscking. Note
that the sync hasn’t taken place until you see the “OK” and “Done” appear
on the screen. (If the kernel is really in strife, you may not ever get the
OK or Done message...)

umount(u) is basically useful in the same ways as sync(s). I generally
sync(s), umount(u), then reboot(b) when my system locks. It’s saved
me many a fsck. Again, the unmount (remount read-only) hasn’t taken place until
you see the “OK” and “Done” message appear on the screen.

The loglevels 0-9 are useful when your console is being flooded with
kernel messages you do not want to see. Selecting 0 will prevent all but
the most urgent kernel messages from reaching your console. (They will
still be logged if syslogd/klogd are alive, though.)

term(e) and kill(i) are useful if you have some sort of runaway process
you are unable to kill any other way, especially if it’s spawning other
processes.

“just thaw it(j)” is useful if your system becomes unresponsive due to a
frozen (probably root) filesystem via the FIFREEZE ioctl.

That happens to me, also. I’ve found that tapping shift, alt, and control
on both sides of the keyboard, and hitting an invalid sysrq sequence again
will fix the problem. (i.e., something like alt-sysrq-z). Switching to
another virtual console (ALT+Fn) and then back again should also help.

There are some keyboards that produce a different keycode for SysRq than the
pre-defined value of 99
(see KEY_SYSRQ in include/uapi/linux/input-event-codes.h), or
which don’t have a SysRq key at all. In these cases, run showkey-s to find
an appropriate scancode sequence, and use setkeycodes<sequence>99 to map
this sequence to the usual SysRq code (e.g., setkeycodese05b99). It’s
probably best to put this command in a boot script. Oh, and by the way, you
exit showkey by not typing anything for ten seconds.

In order to register a basic function with the table, you must first include
the header include/linux/sysrq.h, this will define everything else you need.
Next, you must create a sysrq_key_op struct, and populate it with A) the key
handler function you will use, B) a help_msg string, that will print when SysRQ
prints help, and C) an action_msg string, that will print right before your
handler is called. Your handler must conform to the prototype in ‘sysrq.h’.

After the sysrq_key_op is created, you can call the kernel function
register_sysrq_key(intkey,structsysrq_key_op*op_p); this will
register the operation pointed to by op_p at table key ‘key’,
if that slot in the table is blank. At module unload time, you must call
the function unregister_sysrq_key(intkey,structsysrq_key_op*op_p), which
will remove the key op pointed to by ‘op_p’ from the key ‘key’, if and only if
it is currently registered in that slot. This is in case the slot has been
overwritten since you registered it.

The Magic SysRQ system works by registering key operations against a key op
lookup table, which is defined in ‘drivers/tty/sysrq.c’. This key table has
a number of operations registered into it at compile time, but is mutable,
and 2 functions are exported for interface to it:

register_sysrq_key and unregister_sysrq_key.

Of course, never ever leave an invalid pointer in the table. I.e., when
your module that called register_sysrq_key() exits, it must call
unregister_sysrq_key() to clean up the sysrq key table entry that it used.
Null pointers in the table are always safe. :)

If for some reason you feel the need to call the handle_sysrq function from
within a function called by handle_sysrq, you must be aware that you are in
a lock (you are also in an interrupt handler, which means don’t sleep!), so
you must call __handle_sysrq_nolock instead.

When I hit a SysRq key combination only the header appears on the console?¶

Sysrq output is subject to the same console loglevel control as all
other console output. This means that if the kernel was booted ‘quiet’
as is common on distro kernels the output may not appear on the actual
console, even though it will appear in the dmesg buffer, and be accessible
via the dmesg command and to the consumers of /proc/kmsg. As a specific
exception the header line from the sysrq command is passed to all console
consumers as if the current loglevel was maximum. If only the header
is emitted it is almost certain that the kernel loglevel is too low.
Should you require the output on the console channel then you will need
to temporarily up the console loglevel using alt-sysrq-8 or:

echo 8 > /proc/sysrq-trigger

Remember to return the loglevel to normal after triggering the sysrq
command you are interested in.